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The UET Advanced Vanadium Flow Battery – the Uni.System™ – provides critical buffering capabilities between bulk energy generation and sustainable energy use.

Energy generated from inherently variable renewables, as well as from other grid resources, is stored and then when optimal for the grid and/or at the micro-grid level, released for use.

This buffering can be in multitudinous time scales, from less than a second to hours to days, and with multiple applications run by the control system.

 


The Science

Redox Flow Battery (RFB)

The RFB is a unique electrochemical device that stores electrical energy in liquid electrolytes, instead of in electrodes as many other batteries do. The RFB then releases the stored energy according to the demands of the customer, at levels up to multi-MW and MWhs. As shown in Figure 1, a RFB cell consists of two electrodes or “stacks” (made from carbon felt and other items) and typically two circulating electrolyte solutions (a positive/cathode-side electrolyte or catholyte, and a negative/anode-side electrolyte or anolyte) that are separated by an ion exchange membrane or a separator. The energy conversion from electricity energy to chemical potential energy (charge) and vice versa (discharge) occurs instantly within the electrodes as the liquid electrolytes flowing through the cell.

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Figure 1: Schematic illustration of the structure of a redox flow battery

Because of this structure, RFB’s allow for separate design of energy (kWh) and power (kW) and the power/energy ratio can be tuned according to the applications wanted by the customer. During operation, the flowing electrolytes carry away the heat generated from the electrode reactions and ohmic resistance such that the electrolyte tanks act as a large heat sink, preventing overheating of the battery stacks and the individual cells within them). The physical separation of cells/stacks and electrolytes also helps avoid thermal run-away as may happen in other batteries. The RFB’s generally safe profile is also enhanced by the fact that operation can stopped at any time (including any emergency) by turning off the pumps. There is only a limited volume of electrolytes left in the stack cells where self-discharge may occur. Also, the elimination of repeated ion insertion and de-insertion in electrodes as occurs in other batteries, preserves the structural and mechanical integrity of the cells/stacks, enabling a long-cycle life of the battery. In a true RFB that utilizes liquid electrolytes on both positive and negative sides, its cycle life is generally independent of the battery’s stage of charge (SOC) and depth of discharge (DOD). This is not the case with non-RFB batteries that store energy in their solid electrodes.

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Vanadium Redox Flow Battery (VFB)

The RFB may be traced back to Zinc-halide batteries that use “consumable” Zn/Zn2+ anodes. True RFBs were first used in the early 1970’s by Dr. Thaller’s research group at NASA with Iron-chrome (Fe-Cr) chemistries that utilized “inert” electrodes and had both reactants and products dissolved in liquid electrolytes. To avoid the cross-contamination and improve electrochemical activity over the early Fe-Cr RFB, the all-vanadium RFB or VFB was first demonstrated by Professor Skyllas-Kazacos’s research group in 1980’s at the University of New South Wales. As shown in Figure 2, the VFB utilizes a V2+/V3+ aqueous sulfate solution on the anode side and a V4+/V5+ aqueous sulfate solution on the cathode side. A standard voltage of 1.25 V is generated by the VFB through the following reactions:

Cathode: technology clip image002

Anode: technology clip image002 0000

Cell: technology clip image002 0001

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Figure 2: Schematic of all-vanadium redox flow battery, in a charge process.

The VFB demonstrates an excellent electrochemical reversibility and virtually an unlimited cycle life. Over 10,000 full cycles (from 100% SOC) has been demonstrated in the field. The use of aqueous-base catholyte and anolyte further strengthens the safety record of VFB’s. The favorable general features of RFBs and in particular the advantages with the VFB have attracted wide interests in using the VFB for utility and other large scale stationary applications.

However, the traditional VFB is limited in the stability of its basic electrolyte chemistries. Vanadium oxides, e.g. V2O5, tend to precipitate out from the electrolytes via irreversible reactions, resulting in capacity loss and degradation in the battery’s durability and reliability. As such, the operation of the traditional VFB is limited in a temperature range to 10 ~ 40 °C (practically, 35oC), imposing the burden of heat management and energy loss due to heat management. Additionally, the limited stability of the traditional VFB electrolytes at elevated temperatures limits vanadium solubility in the electrolytes and as thus energy capacity or density of VFB’s, a disadvantage compared with many other batteries.

UET New Generation VFB

To improve performance, reliability and economics of VFB’s, UET has licensed and utilizes a new generation electrolyte chemistry initially developed at the US Department of Energy’s (DOE) Pacific Northwest National Laboratory (PNNL), with the support of the US DOE Office of Electricity Delivery and Energy Reliability’s Grid Storage Program. (Please see: “A Stable Vanadium Redox-Flow Battery with High Energy Density for Large-Scale Energy Storage,” Li, Kim, Wang, Vijayakumar, Nie, Chen, Zhang, Xia, Hu, Graff, Liu, Yang, Ad Energy Mat., Volume 1, pages 394–400, May, 2011, http://onlinelibrary.wiley.com/doi/10.1002/aenm.201100008/abstract.) This new electrolyte is a breakthrough in energy storage technology and received the Federal Laboratory Consortium (FLC) award in 2012. The new vanadium electrolytes with a sulfate-chloride based complex chemistry were proved to have a much improved stability over the traditional sulfate based chemistry. Negligible Cl2 and other gas evolutionduring operation up to 100% SOC has also been proven. Meanwhile the UET RFB (see Figure 3) with the new generation electrolytes demonstrates a voltage and reversibility that is at least as excellent as that of the traditional VFBs with the sulfate electrolytes, via the following reactions:

Cathode: chemistry1

Anode: chemistry2

Cell: chemistry3

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Figure 3: Schematic of UET VFB, during a charge process.

With the much improved stability of the new electrolytes and optimized cell/stack design and electronic controls, UET VFB demonstrates the following features:

  • A practically doubled energy density due to a higher solubility and a higher utilization of the active vanadium species;
  • Operation in temperatures up to >50oC and down to lower than -40oC), easing heat management, mitigating solid vanadium oxide precipitation and capacity fading, and improving energy efficiency;
  • No oxygen evolution (chlorine would come out first, if any) on the graphite felt cathode electrodes during any overcharging (even locally), avoiding oxidation of the graphite electrode and degradation in the electrode reactivity, and thus raising the voltage up-boundary and electrolyte utilization;
  • Advanced balancing management between cathode- and anode-side electrolytes due to a much higher vanadium concentration allowed (>3M) at the cathode side;
  • Improved tolerance to impurities in the electrolytes due to the chemistry changes over the traditional sulfate systems;
  • Improved battery system reliability and durability due to the all aforementioned.

To fully realize the potentials of the new generation VFB and maximize its value to customers, UET has developed and is producing container based products that integrate novel system designs and power electronic control.

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